July 13, 2013

Why so few big thunderstorms in the Northwest?

Day after day this time of the year there are stories of big thunderstorms and severe convection in the eastern two-thirds of the U.S.  Tornadoes, hail, flash floods, supercell thunderstorms, squall lines, and most recently derechos.   Cumulonimbus clouds that can get to 40, 50 or 60 thousand feet or more.

In comparison, the Pacific Northwest is a thunderstorm and severe thunderstorm desert.  Below is a map showing the number of days with thunderstorms a year.  Zero to 10 days  (gray color) over Washington and coastal portions of Oregon and California.  Kind of sad.

 Not only do we get few thunderstorms, but severe thunderstorms are exceedingly rare in our region.  Here is a map showing the number of severe thunderstorm watches released by the National Weather Service for 1999-2008.  Very, very few for the West Coast.

 Northwest thunderstorms are rather pathetic creatures, with low tops. It is so bad that Northwest meteorologists get excited if one gets to 20 thousand feet.  To illustrate, here is a graphic showing the current thunderstorm tops around the U.S. (10 PM PDT Saturday). Nothing going on in our neck of the woods, but tops of 50,000 and 65,000 ft  over the eastern two-thirds of the nation. (650 means 65,000 ft, 300 means 30,000 ft, etc.)

Our thunderstorms are infamous for sparse lightning (one boom wonders) and only rarely possess rotation.  No storm chaser companies will ever set foot in our region. 

Who or what can we blame for our thunderstorm weakness?
The answer is clear:  the Pacific Ocean.

Thunderstorms thrive when the environment has a large decline of temperature with height (called the lapse rate in the weather biz) and the atmosphere has a lot of moisture.

Let's take lapse rate...ours are generally not that large.  First, the Pacific Ocean sea surface is quite cool (about 50F in summer) and our air is generally coming off the ocean.  So we start with the lower part of the atmosphere being cool.  Not good.  Then we generally have high pressure over the eastern Pacific in summer (the East Pacific High) and that is associated with descending air and warm temperatures in the middle portion of the troposphere (the lowest layer of the atmosphere, from roughly 0 to 30,000 ft).    With cool temperatures near the surface and warm air aloft, it is hard to get a large decrease of temperature with height over our region during the summer.  Strike one for thunderstorms.

Thunderstorms are also helped by having lots of moisture in the lower atmosphere.  Why?  When air becomes saturated and rises, heat is released (called latent heat) and this heating makes the air more buoyant in the thunderstorms. Now the amount of moisture that air can contain depends on temperature (warmer air holds more water vapor).    Well, the Pacific and the air next to it are quite cool; thus the air can't pick up much moisture!  Strike two for thunderstorms.

Really big, severe thunderstorms also need large vertical wind shear (large change in wind speed or direction with height).  Wind shear typically is fairly modest around here during the summer, since high pressure dominates our weather.  Strike three for thunderstorms.

Thus, for a number of reasons, our thunderstorms are the 98 lb weaklings of the weather world.  But the eastern U.S. folks should not get too smug, during the winter our weather bulks up!


In a recent blog I talked about the importance of GPS weather satellite like COSMIC and the fact that NOAA and Congress appears to turning down the chance to work with Taiwan to launch the next generation of these satellites, with Taiwan paying for half the bill.  Really bad move--we critically need the data.  You can ask your representatives in Congress to support COSMIC-2 by going to this web site and providing your name and email.   Thank you...its important.


  1. I'll take subduction zone earthquakes, impending volcanic activity and the most boring weather ever, over tornadoes, hurricanes and derechos, any day of the week. On the other hand derechos have got to be one of natures most interesting phenomenons especially when seen over Minnesota. A place one would not expect to see any significant thunderstorm activity.

  2. So imagine what it would take to create the mother of all thunderstorms around here? Exceedingly rare yes, but not impossible.

    I remember the storm in 1984 or 1985, which produced that great photo of downtown Seattle with the huge lightning strike. Another in 1999 lasted for hours and had thunder strong enough to set off car alarms.

    What would have to come together to produce an unbelievable storm in this area? I'm sure it would be interesting!

  3. The biggest thunderstorm I remember in recent times was the summer of 2000. It was not characterized by intensity, but by scattered frequent strikes from every direction and distance and for an extended time. And the next night it happened again but not so dramatic. I don't have the exact date but come up with it should someone want it. Rob

  4. I live in the Walla Walla Valley and have noticed greater-sized cloud formations above the plateau between Walla Walla and Pendleton in recent years. Little local rain and lots of thunder.

    Lots of heat lightening recently during otherwise clear nights.

  5. So, I guess this lets us out from getting any Sharknadoes her then eh?

  6. 2003 seemed to have a big one. Started around 3:30 am here in Duvall, and didn't really stop until the evening afterward. Just kept going. Then, there were the two storms in late June and early July 2008. And, don't forget the storms a year ago.

  7. I know in summer that thunderstorms like to form in The Cascade Mountains because of the orographic lifting that is caused by the occasional summer time front that moves through but other then that not much in the way of extreme weather. However I have been to Lassen and the tops I saw in some of those storms got to 30,000 feet at one point and got a severe thunderstorm warning for large hail. Most of our storms when severe warned are either a hail or flash flood threat since they go up rapidly and then collapse due to little to no wind shear and dump all of their precipitation in one area. Other then that they can be a wildfire threat due to the chances of dry lightning.


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